This invention pertains to a hydraulic lifting mechanism for an article of furniture wherein a hydraulic power unit is provided for raising and lowering a retractable support member for supporting a chair seat or the like.
It is desirable in certain articles of furniture, such as chairs, to provide a mechanism for adjusting the height of a supporting member as for example the height of a chair seat whereby the chair occupant can select a height of the seat best suited for him. Prior art structures have been provided for this purpose and those structures have in general comprised a pair of rigid walled hydraulic fluid filled chambers which communicate with each other by means of a hollow conduit. One of the chambers is expandable so as to have a variable volume and the other chamber contains a compressed gas whereby the hydraulic fluid can be transferred from the pressurized chamber to the expandable chamber when a control valve in the hollow conduit is opened. As the fluid volume in the expandable chamber increases, the support member will be urged upwardly, thereby raising the chair seat. Control means are provided to control the hollow conduit valve to enable transfer of fluid between the chambers. The chair seat is returned to its retracted position by transferring fluid from the variable volume chamber to the pressurized chamber. As the expandable chamber is compressed the volume of that chamber is reduced thereby causing the fluid to be forced out of that chamber and into the pressurized chamber.
One prior art patent showing the structure hereinabove described is U.S. Pat. No. 4,074,887. This patent shows two circumferentially arranged chambers with the outer chamber comprising a rigid walled sealed chamber containing hydraulic fluid and containing a pressurized gas in a top portion thereof. The inner chamber is a rigid walled expandable chamber containing only hydraulic fluid. A control lever is provided for controlling a valve to enable hydraulic fluid to be transferred by means of the gas pressure from the outer chamber to the inner chamber. A piston in the expandable chamber is raised and lowered by operation of the control valve and transfer of the hydraulic fluid between the chambers.
A disadvantage of this prior art structure is that the sliding seals needed to seal the piston in the chamber are subject to wear and leakage and, as hydraulic fluid leaks out of the mechanism, the volume of hydraulic fluid in the mechanism is reduced. The pressurized gas in the outer chambers will then occupy a greater volume, thereby reducing its pressure and causing the upward speed of the mechanism to decrease and the upward force on the support and chair seat to be reduced, both of which results are undesirable. Furthermore, the lost hydraulic fluid needs to be replaced from time to time so that the mechanism requires servicing, which is undesirable. It is therefore desirable to provide a pneumatic lifting mechanism wherein no sliding seals are provided and which is not subject to leakage and loss of hydraulic fluid.
A further disadvantage of the prior art structures has been that the control for adjusting the lifting mechanism has been located adjacent to the mechanism. The prior art arrangements have necessitated the occupant of the chair to lean over and reach far under the chair seat, or have necessitated long control arms to adjust the chair height. It is desirable to provide a lifting mechanism wherein the control can be located in the chair arm or adjacent the seat so that it is more easily operable.
A yet further disadvantage of the prior art structures has been the provision of rather complicated valves to control the transfer of hydraulic fluid between the chambers. It is therefore desirable to provide a mechanism including a valve which is simple yet reliable and effective.
U.S. Pat. No. 4,445,671 discloses an adjustable pneumatic mechanism for an article of furniture which prevents rapid dropping of the support platform. A pressure sensitive valve is provided which allows fluid to flow in one direction, but which prevents fluid flow in the opposite direction if excessive pressures are encountered in the hydraulic mechanism. Thus, if a heavy weight is placed on the platform, causing a high pressure to be developed in the hydraulic fluid, the valve will shut and prevent rapid descent of the mechanism and potential injury to the operator. The disadvantage of this structure is that the valve mechanism is rather complicated. It is therefore desired to provide a lifting mechanism wherein a simple and reliable control valve allows for a larger rate of fluid flow in one direction than in the reverse direction so that the ascent of the hydraulic mechanism is accomplished at a rapid rate, whereas the descent of the mechanism will occur much more slowly, thereby preventing injury to the operator and also permitting the operator to stop the descent of the mechanism at the exact desired point.
In some of the prior art pneumatic lifting mechanisms, only a gas rather than an incompressible hydraulic fluid is used to provide the lifting functions of the mechanism. These types of mechanisms are subject to leakage of gas from the mechanism and also result in a spongy action of the mechanism since the gas is compressible. It is desired to provide a very positive lifting mechanism not subject to leakage of the working fluid or having spongy action and wherein the hydraulic working fluid is incompressible.
In still other prior art mechanisms a manual or electric pump is provided to pump hydraulic fluid to the expanding chamber to cause the lifting action. Such pumps are subject to failure and are also costly. It is therefore desirable to provide a mechanism wherein no pumps are needed to transfer fluid between the two chambers.